This invention relates to a method of molding composite, structural plastics and the objects fabricated thereby. In particular, this invention relates to a method of casting plastic commercial components in conventional metalcasting molds without the need for injection or compression molding. This invention further relates to the rapid fabrication of prototypes in composite, structural plastics using conventional soft tooling or rapid prototyping techniques.
The use of molds to create parts of varying size, quality and implementation pervades the industrial landscape. Over 4000 metal casters cast over 32 billion pounds of metal annually in metal foundries all over the world. In the foundries, metals are processed into commercially viable shapes by melting and pouring a molten metal into a mold. In this manner, structural items can be fabricated from steel, iron, copper, aluminum and like materials for a virtually limitless variety of applications.
Selection of a specific metalcasting process depends upon several factors, including, without limitation, the metal or alloy selected, casting size and complexity, surface finish, dimensional tolerances, production quantities and cost constraints. In addition, selection of a mold must anticipate whether the process uses expendable molds which are used only once and then discarded (i.e. in sand casting operations) or metal molds intended for repeated use (i.e. as used in permanent molding and diecasting). No matter what metalcasting process is used, however, all processes share two main objectives: the pattern must be removable from the mold without damage, and the casting must be removable from the mold or die without damage to either of the die or the casting (see 1996 CDandA Reference handbook). Various metalcasting processes are described hereinbelow.
1. Conventional Metalcasting Processes:
a. Sand Casting
Sand casting metal is the backbone of several predominant industries, such as the automotive industry, because the materials and the tooling used in the process are inexpensive and rapidly produced. More than 80% of all castings made in the United States are produced by green sand moldings (see 1996 CDandA Reference handbook). The term xe2x80x9cgreen sandxe2x80x9d denotes a mixture of raw sand and a binder that has been tempered with water.
Sand molding is a multipurpose metal-forming process in which a pattern is made of wood, metal or plastic based upon the design specifications of the casting. In a conventional sand casting process illustrated in FIG. 1A, a pattern 10 is usually constructed in two parts which include a bottom part 10a and a top part 10b to allow ease of pattern removal from a mold. Parts 10a and 10b are aligned with each other using a plurality of registration pins 13.
Referring to FIG. 1B, bottom part 10a of the pattern is placed upside down on a molding board 16. In this way, the pattern defines a desired shape within a bottom half 18a of a mold 18. The bottom half 18a of mold 18 is then filled with green sand 19 as shown in FIG. 1C. Sand 19 is compacted firmly around and over the pattern by manual or mechanized compression means, such as a ram 21. The bottom half 18a of mold 18 is then inverted and set on a board or pallet 24, and molding board 16 is removed therefrom, as seen in FIG. 1D. Top part 10b of the pattern is aligned with bottom part 10a and set using a plurality of alignment pins 26. The separation of the pattern parts defines a recognizable parting plane 27 therebetween such that a shape is defined in the top half 18b of mold 18 that is substantially symmetrical to that defined in the bottom half .
As shown further in FIG. 1E, the top half 18b of mold 18 is filled with sand 19 which is then compacted over and around the top part 10b of pattern 10 with ram 21. A vertical channel or sprue 27 is cut into the top half 18b of mold 18 to provide an ingress for pouring molten metal into a mold cavity. Mold 18 is then parted along parting plane 27, and pattern 10 is removed therefrom.
As depicted in FIG. 1F, a horizontal channel or runner 29 is cut in the lower half 18a of mold 18 so as to be in communication with sprue 27 to accommodate flow of molten metal therethrough. If it necessary to compensate for metal shrinkage during the process, one or more risers 31 can also be cut into the mold. As further illustrated in FIG. 1G, a sand core 33 is set in place and positioned using core prints 14 that are created in the mold by pattern 10 (shown in FIG. 1A). Mold 18 is finally closed thereafter and ready to produce a casting.
Sand casting can be used to mold a wide range of materials having considerable complexity. The low tool and die costs associated with this method, coupled with the ability to produce varying lot sizes of materials (i.e. a few pieces or huge quantities can be produced) make sand casting desirable for a wide range of applications. However, a significant disadvantage of this type of molding process is that the mold is a single use mold which inhibits high volume production. Furthermore, the use of binder within the sand anticipates the release of toxic substances into the environment upon removal of the binder and disposal thereof. The low tool and die costs are compromised by high labor and finishing costs which are incurred during the production cycle.
b. Permanent Molding
In permanent mold casting (also known as xe2x80x9cgravity diecastingxe2x80x9d), a metal mold consisting of at least two parts is repeatedly used for components that require high volume production. A conventional permanent mold arrangement is illustrated in FIG. 2. Up to 99% of such molds currently in use are made of steel or plaster; however, these mold molds may also be constructed of cast iron, graphite, copper or aluminum.
Molten metal is poured into a mold 41 having mold halves 41a and 41b and a core 43. Mold 41 is a permanent mold wherein the metal cools more rapidly than in a sand mold and produces a finer grain structure with enhanced mechanical properties and tighter dimensional tolerances. As can be seen from FIG. 2, mold 41 emulates the sand casting procedure described hereinabove, except that a material such as metal or plaster is used in place of sand. In this process, a mold configuration 44 is formed in the mold which corresponds to the desired configuration of the cast product. A sprue 45 is defined for pouring of molten metal into the mold cavity defined by mold configuration 44.
Although the permanent mold process has moderate labor costs and low finishing costs, the problems associated with this procedure include limitations on casting size coupled with high initial tooling costs, which make the process prohibitively expensive for low production volumes. In addition, several alloys and shapes are not amenable to permanent mold casting due to part line location, complex undercuts in the design or difficulty in removing the casting from the mold. Lot size is limited to large quantities, making the process untenable for small scale molding. Furthermore, mold coatings which are often required to protect the mold from erosion, cracking and other forms of metal degradation can deleteriously effect surface finish.
c. Diecasting
Diecasting is a permanent molding process is primarily for high production of intricately-designed components cast from zinc, lead, tin, aluminum, copper or magnesium. There are two types of diecasting machines: cold chamber (illustrated in FIG. 3A) and hot chamber (illustrated in FIG. 3B). In either method, a molten alloy 51 is manually or automatically poured into a shot well 53A or 53B and injected into a die 55A or 55B under pressure. The locking force in diecasting machine operation keeps the die halves firmly closed against the injection pressure exerted by a plunger 57A or 57B as the plunger injects the molten metal.
As further shown in FIG. 3A, during cold chamber diecasting, molten metal is held at a constant temperature in shot well 53A prior to high-pressure injection thereof by piston 59A into die 55A. The cold chamber method is primarily used with metals of higher melting temperatures, such as aluminum and magnesium. Conversely, as shown in FIG. 3B, during hot chamber diecasting, molten metal is held in a temperature-controlled holding pot 58 and automatically discharged through a port 56 located at the top of shot well 53B. Discharges occur between each high-pressure injection of the molten metal to die 55B. The hot chamber method is primarily useful with those metals having low melting temperatures, such as zinc alloys.
Either form of the diecasting process allows part designers to use complex designs and cast-in inserts of other materials, such as steel, iron, brass and ceramics. However, the material used for the cast components themselves is limited to a narrow choice of materials such as zinc, aluminum, brass and magnesium. This method, those most economical where applicable, includes high tool and die costs and is only practical for production of large quantities of components.
d. Investment Casting
In an investment casting process, shown in FIG. 4, a ceramic slurry is poured around disposable pattern typically formed of paraffin waxes or plastics. The slurry is allowed to harden to form a disposable mold, and the pattern is destroyed upon melting during the firing of the ceramic mold. Later, molten metal is poured into the ceramic mold. After the metal solidifies, the mold is broken to remove the casting therefrom.
Two processes are generally used to produce investment casting molds: the solid mold and the ceramic shell method. The ceramic shell method dominates the use of this production technique and is therefore illustrated as a series of discrete steps in FIG. 4. Wax is injected into an aluminum die to form a pattern that replicates the desired casting configuration (1). For smaller castings, several wax patterns are affixed to a common tree so as to accommodate larger lot sizes (2). The wax components are then dipped into a liquid ceramic slurry (3) and coated with dry refractory sand until a shell is developed thereon (4). The wax is then melted out in a furnace (5) wherein the shell is hardened, producing a single-piece shell mold (6). Molten metal is poured into the ceramic mold (7), and the shell is broken away after the metal has cooled and solidified (8). The solidified metal component may be subjected to further finishing processes (9) and inspected thereafter (10) to assess the quality of the component and its applicable uses.
Because the wax pattern can be made with internal passageways to create complex castings, the investment casting process enables mass production of complex shapes and reproduction of fine details with tighter dimensional tolerances. However, initial tooling costs for larger castings are extremely high, and the size and weight of components which can be produced by this method are limited, imposing escalated time and financial burdens on the manufacturer.
e. Shell Molding Process
In a shell molding process, the steps of which are shown in FIG. 5, a thermosetting resin-bonded silica sand 62 is placed on a heated pattern 65 for a predetermined length of time (1). Heating cures the resin, causing the sand grains to adhere to each other to form a sturdy shell that constitutes one-half of a thin-shelled mold 66 (2). Upon ejection of the pattern 65 from the shell 66 (3), the shell is manually joined with its complementary other half to make a complete shell mold 68 (4).
Castings made by this method typically exhibit more accurate dimensional tolerances than conventional sand castings with a high degree of reproducibility. A wide choice of materials can be used in this process for the production of moderately complex designs, with the exception of low carbon steels. Tool and dies costs are low, yet the process requires larger lot sizes to be practicable. Moderate-to-high labor and finishing costs are also associated with this method.
f. Lost Foam Casting
Lost foam casting is also known as expanded polystyrene (EPS) molding, expendable pattern casting, evaporative foam casting, the full mold process, the cavityless casting process and the cavityless EPS casting process. In a lost foam casting process, the steps of which are shown in FIG. 6, a one-piece pattern 71 is made of expanded polystyrene and covered with a thin refractory coating (1). Pattern 71 is embedded in unbonded sand 72 within a vented container 73 (2). Molten metal that is poured into a sprue 73a vaporizes the polystyrene instantaneously (3), quickly reproducing the pattern to form a finished product 78 (4). Gases 76 which are formed from the vaporized pattern escape through the pattern coating, sand 72 and the vents of container 73.
This process is advantageous in that is requires no cores and enables production of complex, close-tolerance castings with near net shape. Furthermore, castings can be made by the lost foam process with no parting lines and with a substantial reduction in capital investment and operating costs. However, pattern handling requires considerable care, resulting in labor costs that are very high and further limiting the types of materials that can be used.
2. Selection of Metalcasting Process Methods
Molding systems and casting processes other than those described hereinabove are used to make metalcastings, such as vacuum molding and use of centrifugal casting machines. However, when all of these processes are considered together, no one process emerges as a dominant low cost production method suitable for casting a wide array of configurations. Furthermore, no one method enables easy transition among production objectives. The employed processes must often be changed due to a change in volume production or material selection, even if other casting specifications remain the same. The inability to employ a single system or process to produce a broader spectrum of castings maximizes operating and maintenance costs, especially if the required specification and volume cannot be met by the system or process that is already in place. Generally, specialization always leads to higher costs and lengthier production times, and the same is true with traditional metalcasting methods.
3. Modem Substitution of Plastics
In light of the problems associated with conventional metalcasting procedures, injection and compression molders and fiber reinforced plastics have become increasingly utilized in the production of articles once exclusively made through such procedures. Both injection molding and compression molding processes have been used to provide fiber-reinforced equivalents of metal objects. In both of these processes, resin and a reinforcing fiber are combined and formed into a shape that can be molded. While the material is in the mold, high added heat melts the resin and ensures a complete transition into a fully cross-linked and cured polymer. This high added heat is applied in concert with sufficient pressure to force the material into a mold. Typical temperatures reach ranges of 250-650xc2x0 F. and typical pressures reach 150-5000 psi.
An increasing number of businesses are molding composite structural plastics to produce objects in place of equivalent metal parts. Molding the plastics to achieve net shape, weight reduction, corrosion resistance and reduced energy costs is desirable in many industries not only to reduce production costs but also to improve performance of the molded objects. As a result, the metalcasting industry is slowly, yet definitively losing ground to more modern synthetic materials and the use thereof in a wide variety of industrial applications.
a. Prototyping
In view of the far-reaching and advantageous application of modern plastics, many industries, including the automotive industry, covet the ability to bring a product from conception to full-scale product development in the shortest time span. In most cases, this involves an early step of producing at least a non-functional, visual display prototype of the object to be manufactured. Prior to recent computer developments in prototyping, wood forms would be machined to provide the form of the object so that a wax, plastic or rubber pattern could be made quickly in order to produce at least a handful of three-dimensional models prior to manufacture. Such models have always been unfilled plastics, incapable of serving as structural prototypes.
Today, computer-aided design (CAD) is frequently employed for at least rapid visualization of an article to be manufactured. While enormously useful to engineers studying the best production methods for the object, CAD has been further improved so as to actually produce a three-dimensional object for handling, visualization and limited suitability testing. These CAD techniques include stereo lithography (SLA), laminated object manufacturing, selective laser sintering (SLM), fused deposition modeling (FDM) and solid ground curing (SGC). These techniques use powder, liquid or sheets of polymers or other materials which are sequentially formed together, eventually producing a prototype of the desired object. Hereinafter, all of these CAD techniques are collectively referred to xe2x80x9crapid prototyping techniquesxe2x80x9d.
For virtually all of the prototype techniques, including conventional soft tooling and state-of-the-art CAD prototype production methods, the result is a prototype with relatively low temperature resistance and strength. While extremely useful at the early visual stage of product development, these prototypes cannot be used to fully evaluate the functionality of a finished product. A typical example of current prototype fabrication is found in the automotive industry. If a prototype for a nylon intake manifold is required, for example, an initial model would be made using a rapid prototyping technique such as SLA. However, to test the functionality of the prototype, automotive design engineers then have to make a steel mold and inject nylon to produce the same design in plastic so that the prototype will withstand high temperatures and stresses. In effect, the designer is repeating the prototyping procedure at great expenditures of money and time.
b. Industrial Applications
The automotive industry is a primary example of the inevitable conversion from metalcasting to plastic part production to optimize performance of molded parts while reducing the production costs associated therewith. Thus, throughout this specification, reference will be made to this ubiquitous industry. However, it is understood that the present invention methods and systems, and the products rendered thereby, are amenable to a limitless number of applications in a multitude of industries around the world.
Molded nylon intake manifolds, for example, already constitute 5% of the relevant market, and manufacturers have indicated that such manifolds will eventually replace the conventional aluminum designs currently in use. However, the process of conversion from aluminum to plastic production has been slow because of the high cost for prototype and production injection molding tooling and associated process equipment. The conversion to plastic requires production of new, custom-made steel molds which are prohibitively expensive for most producers. Therefore, a molder must wait until the automaker allocates a budget to pay for this very expensive tooling.
During the average development program, it takes 10-12 weeks to produce a steel mold. Due to the high temperatures (i.e. 500-600xc2x0 F.) and pressures (i.e. 3000-5000 psi) required with nylon injection molding, molds must be machined out of tool steel. Along with the development of the manifold has been the development of a method to produce the intricate internal passages previously created with sand cores when casting manifolds in aluminum. The method used with injection molding nylon is known as the lost core process. It uses a soft metal such as tin-bismuth as the soluble core material. The metal is first melted, and then injection molded in a steel mold to produce the core. A steel mold is required due to the high melt temperature of the metal and the stress it places on the mold material when injected. The average development program consumes three steel injection molds for the nylon manifold and three steel molds to mold the expendable tin-bismuth cores. Costs for the development program alone can exceed $1 million just for prototype tooling.
Since inception, injection molded manifolds have used the lost core process to provide rather simple manifold design configurations. As engineers have embraced nylon injection molding, the complexity of the designs and their core packages have increased significantly. Engineers and molders have experienced extreme difficulty not only in preventing shifting of the tin-bismuth cores, but also in melting out the cores, thereby creating a crisis for the nylon manifold business and the automotive industry. The current manifold designs are so complex that the core package weights are extremely dense and heavy. A popular V8 engine manifold, for instance, has 160 lb of metal in its core package, causing the core to shift in the mold as a result of its own mass. This anomaly is further aggravated by the high injection pressures of the nylon molding compounds.
In addition, since nylon cannot be injection molded in thick cross-sections, the injection molded manifolds can only have thin wall thicknesses (i.e 3-4 mm), significantly less than the thicknesses of a conventional sand cast aluminum manifolds (i.e 6-10 mm). The cost of the injection molded materials also mandates thin wall designs. As a result, injection molded manifolds are much noisier than their aluminum counterparts in that they do not dampen the noise, vibration and harshness (NVH) generated by the engine as well as cast aluminum manifolds.
c. Comparison With Metalcasting
Injection molding plastics are similar in costs to diecast metals such as magnesium, yet magnesium and most structural injection-moldable plastics cost twice as much as cast aluminum. It is usually felt that injection molding is the lowest cost form of molding plastics and that casting is more expensive. Sand casting, however, is the slowest method of casting metals from a cycle-time standpoint, but the tooling is the least expensive. Diecasting is the fastest curing method, but the tooling is significantly more expensive and has a short life. In the past, where plastics have been cast, they always required the use of steel molds which is what prevented them from being costs effective with injection molding. Therefore, the need has been felt for a process which integrates the benefits of conventional metalcasting methods and the inherent performance advantages of plastics.
Being able to cast structural plastics using non-traditional plastic tooling has advantages over metalcasting aluminum, particularly in the rapidity of tooling production, the long life of the tooling and the significant cost savings in both the tooling and the materials. Cycle times for such a method of casting plastics are comparable to diecasting aluminum and injection molding plastics. In addition, the metal caster realizes significant cost savings by reclaiming the ability the ability to make the plastic compound in-house. For example, the ingredients which comprise a typical glass-reinforced nylon compound (i.e. the monomer, the catalyst, the fiber, etc.) if purchased separately, can result in a cost of $0.80-$0.90 per pound of compound. In comparison, a typical injection moldable grade nylon currently used to mold manifolds, already mixed and made available as a ready-to-use compound, can cost $1.25-$1.75 per pound of compound. Therefore, if cure of parts could be achieved as quickly as injection molding in molds that are as inexpensive as those used in sand casting or which can be rapidly fabricated at a lower cost than machined steel molds, then a low-cost method of producing plastic equivalents of metal casted objects would be obtained.
a. Prior Art Solutions
The desire to use more varieties and amounts of plastic materials arises from this need to use lower cost methods while retaining the structural integrity of the products made thereby. Plastics are amenable to fabrication in simple and complex forms, enabling volume production of various industrial components. An example is disclosed in commonly assigned U.S. Pat. No. 4,848,292 (the ""292 Patent), which is incorporated by reference herein. The ""292 Patent discloses a cylinder head and engine block assembly for an internal combustion engine wherein both the cylinder head and engine block are formed from a fiber-reinforced phenolic resin. The fiber reinforcement preferably includes fiberglass or graphite fibers having a length of about xc2xdxe2x80x3 to 1xe2x80x3. The head and block are either injection or compression molded to achieve close tolerances, enhanced structural integrity and elimination of most secondary machining operations. In either process, dry resin powder and a reinforcing fiber are pre-mixed and formed into a shape that can be molded. Where injection molding is used, the phenolic molding compound is injected into the mold cavity at injection molding temperatures and pressures to fill the cavity and molding chamber.
A lightweight engine block and head assembly according to invention reduces overall engine weight by up to 60%, reduces noise, minimizes rust and corrosion and significantly reduces the duration and cost of manufacture by reducing the number of secondary machining operations that must be performed to give the assembly its finished shape (i.e. post-mold drilling of bores to accommodate correspondingly sized stud bolts). Composite molding times are significantly shorter than times required for conventional metalcasting, promoting mass production thereof. Additionally, the components are capable of maintaining higher horsepower for their weight than conventional metal parts. Such engines further effectively maintain their shape, dimensional stability and structural integrity at high operating temperatures and have a greater strength-to-weight ratio than metal.
Thus, the combination of a high mechanical strength (particularly at operating temperatures), thermal stability, fatigue strength and excellent compressive strength exhibited by structural composite plastic components makes the materials highly desirable for mass production and continued incorporation into mainstream production cycles. Such materials also exhibit excellent resistance to wear, corrosion, impact, rupture and creep, and components fabricated from such materials reliably operate in the presence of engine fuels, additives, oils and exhaust. Since these characteristics are amenable to various other application, manufacturers have identified the need to develop new prototyping and production programs which incorporate fabrication of a multitude of structural plastic designs and exploit their advantageous properties commercially.
b. Present Inventive Solutions
In co-pending and commonly assigned U.S. application Ser. No. 08/714,813, the disclosure of which is incorporated by reference herein, a method for molding composite structural plastics in molds traditionally used in foundries for molding metal parts is described. The same basic method can be employed to produce not only inexpensive prototypes utilizing soft tooling or rapid prototyping techniques, but also to form molds suitable for structural, prototype or final product fabrication. Such an application is disclosed in co-pending and commonly assigned U.S. patent application Ser. No. 08/877,410, which is also incorporated by reference herein. Thus, a low-cost and rapidly developed molded prototype part, or its commercial equivalent, can be used by design engineers not only to visualize the objects in a hands-on three-dimensional representation, but also to test the object in the actual environment to which the finished product is going to be exposed. Extremely important costs savings are realized both in fabrication of the prototype and in time saved in bringing a newly-conceived product to market.
Thus, it is desirable to combine the desirable characteristics of composite materials with conventional, readily available metalcasting procedures and molds to develop and implement a successful method which permits the use of conventional metalcasting molds to fabricate composite, structural plastic prototypes and products thereby. In addition, it is desirable to develop a material and casting process to produce a structural prototype having functional properties equivalent to those of a finished product using current soft tooling or rapid prototyping techniques, thereby reducing design cycle time and concurrently reducing the elapsed time between conception and market introduction
It is therefore an object of the invention to provide a method for molding composite structural plastics in conventional, readily available metalcasting equipment.
It is another object of the present invention to produce a structural plastic component t using conventional, readily available metalcasting equipment.
It is a further object of the present invention to permit molding of composite, structural plastics without the need for high temperature equipment for post-cure cycles.
It is yet another object of the present invention to permit molding of low cost and lightweight composite plastic equivalents to foundry-produced metal objects.
It is still another object of the present invention to reduce the amount of required machining of components after casting.
It is another object of the present invention to provide a method for utilizing soft tooling and modem rapid prototyping techniques in order to fabricate structural, composite plastic prototype parts.
It is still another object of the present invention to provide a simple, low-cost single step method for producing a prototype part having equivalent visual and structural characteristics to the actual part to be co commercially produced.
It is yet another object to reduce the viscosity of flowable resins and combine such resins with short-length reinforcing fibers so as to enable pouring of the combination into a conventional metalcasting mold.
A method for molding composite structural plastic components is disclosed wherein such components are cast from a polymerizable thermoset or thermoplastic composition in a conventional metalcasting mold. In the instant invention, a low viscosity thermoset or thermoplastic composition having reinforcing fibers distributed therein is poured into conventional metalcasting molds, obviating the need for high heats and pressures associated with injection or compression m molding of composite materials as taught in the prior art. Using metalcasting tooling and procedures heretofore used solely in the casting of production metal parts permits xe2x80x9cno pressurexe2x80x9d molding without high added heat. In the case of a thermoset resin, the object to be fabricated is fully cured by the action of a catalyst at relatively low exothermic resin temperatures. In the case of a thermoplastic resin, curing is generally achieved independently high added heat and pressure. With respect to either a thermoset or thermoplastic resin, the resin is brought to a viscosity sufficient to maintain suspension of a plurality of reinforcement fibers therein.
The invention furthermore discloses a method of fabricating high quality composite structural plastics in traditional soft tool molds and molds produced using rapid prototyping techniques. This economical molding technique permits production of quality structural molded plastics utilizing low cost molds heretofore used only in the prototyping of plastic visual aids.